This invention relates to a sewing machine which sews two sheets of cloths so that patterns of two cloths may match with each other.
This kind of sewing machine is known as disclosed in the U.S. Pat. No. 4612867. A first optical sensor for upper cloth and a second optical sensor for lower cloth is provided upstream of a sewing position in the cloth feeding direction, where the first and second sensors detect light intensity signals of patterns of two superposed cloths in synchronization with feeding of the cloths. The light intensity signals of specified frequency according to intervals of patterns are detected, pattern mismatch distance are calculated based on data thereof, and relative feeding amounts of the two cloths are adjusted by a step motor so that the mismatch distance may be decreased to match the patterns.
There exist, however, cloth types having patterns that cannot be matched by the above-described sewing machine since there are many various kinds of cloths, patterns, and sewing types. Whether pattern matching of the cloths is possible or not has to be determined based on an operator's experience by visual observation of a mismatch amount after sewing. As such, the determination on whether pattern matching is possible or impossible has to be made after sewing Further because it requires experience, there was a problem that the determination was difficult. Moreover, even in sewing the cloths having patterns that can be matched, it was impossible to match patterns when the two cloths are set with patterns mismatching to a great degree, when the cloths are pulled and the patterns are greatly displaced due to an operator's carelessness, or the cloths are caught by guide members and can not be fed regularly.